This invention relates to apparatus for feeding workpieces in a multi-station cross-fed press.
In such a press, metal workpieces, which are usually preheated, are subjected to plastic shaping in successive shaping operations, are picked up by a pair of gripper jaws at each shaping station to be moved to the adjacent station and there released.
There is no doubt that the feeding of workpieces constitutes one of the most critical problems in the construction of cross-fed presses. The pairs of cooperating gripper jaws must be capable of seizing large numbers of complicated parts every minute, and of moving them and releasing them at the correct moment in a precise manner.
Special problems arise with hot presses since, in certain circumstances, the heat released in the shaping zone as well as the formation of scale and the coolant spray can have a deleterious effect upon the functioning of the grippers. Efforts have therefore been made to avoid, as far as possible, a direct connection between the grippers and the die-holders of the press, i.e. to arrange the grippers at a distance from the anvil surfaces of the dies.
German Patent Specification No. 918845 discloses mounting gripper arms on two parallel shafts disposed one on each side of the shaping stations. The shafts are longitudinally displaceable and are interconnected by a bridge element so as to be drivable forward and backward in synchronism with the press. The shafts are also rotated by crank arms about their axes in opposite directions, the rotary movement being transmitted to the gripper arms which, depending upon the direction of rotation, either disengage from the workpiece or seize it.
However, since the grippers are moved about an axis during their opening and closing movements, their contact faces describe an arc which requires a relatively large amount of space and is unfavourable as regards friction during the seizing and release of the workpieces.
To avoid these difficulties, it has been proposed in German Offenlegungsschrift No. 2435395 to move the grippers parallel to the plane of the anvil by means of special guide arms during the arcuate movement. This solution, however, suffers from the disadvantage that the parallel movement of the gripper jaws is, for practical purposes, only possible at one point along their path of movement, and that the mechanism required is very complicated and likely to break down. The same is true of the construction proposed in Offenlegungsschrift No. 2538650 in which, although the grippers do move parallel to the front face of the anvil, the distance over which they open varies in dependence upon their distance from the axis of rotation.